My good friend Dominic Alexander e-mailed me this picture, not because he wanted a diagnosis, but rather to know what I would suggest as a treatment plan.
What would your diagnosis and treatment be?
This is an iris cyst. In the horse, unlike the dog, they are generally attached to the iris and, if they grow in size, they can cause the horse to spook as they move around. Here is another view of this one.
In the USA these are generally lasered (see for example Gemensky-Metzler AJ, Wilkie DA, Cook CS. The use of semiconductor diode laser for deflation and coagulation of anterior uveal cysts in dogs, cats and horses: a report of 20 cases. Vet Ophthalmol. 2004:360-8), but here a simple needle inserted in through the ventral cornea under sedation and a local nerve block can serve to deflate the cyst quite well.